Magnetic resonance imaging (MRI) is a method which utilizes the nuclear magnetic resonance phenomenon (NMR) for finding out the local distributions of the nuclear density and nucleus-related NMR properties of an object or the physical and chemical properties having an effect thereon. Said NMR properties include e.g.: longitudinal relaxation (characterized by longitudinal relaxation time T1), transverse relaxation (characterized by transverse relaxation time T2), relaxation in the rotating frame of reference (characterized by relaxation time T1rho), chemical shift, and coupling between nuclei. The physical phenomena having an effect on NMR properties include e.g.: flow rate, diffusion, paramagnetic materials, ferromagnetic materials, viscosity and temperature. Methods and applications of magnetic resonance and magnetic resonance imaging have been described in a number of references: Wehrli FW, Shaw D, Kneel and RJ: Biomedical Magnetic Resonance Imaging, VCH Publishers, Inc., New York 1988, Stark DD and Bradley WG: Magnetic resonance imaging, C. V. Mosby Comp., St. Louis 1988, Gadian DG: Nuclear magnetic resonance and its applications to living systems, Oxford Univ. Press, London 1982, Shaw D: Fourier transform NMR spectroscopy, Elsevier, Amsterdam, 1984, Battocletti JH: NMR proton imaging, CRC Crit. Rev. Biomed. Eng. vol. 11, pp 313-356, 1984, Mansfield P and Morris PG: NMR imaging in biomedicine, Adv. in magnetic resonance, Academic Press, New York 1982, Abragam A: The principles of nuclear magnetism, Clarendon Press, Oxford, 1961, Lasker SE and Milvy P(eds.): Electron spin resonance and nuclear magnetic resonance in biology and medicine and magnetic resonance in biological systems, Annals of New York Academy of Sciences vol. 222, New York Academy of Sciences, New York 1973, Sepponen RE: Discrimination and characterization of biological tissues with magnetic resonance imaging: A study on methods for T1, T2, T1rho and chemical shift imaging, Acta polytechnica scandinavica EL-56, Helsinki 1986, Fukushima E and Roeder SB: Experimental pulse NMR, Addison Wesley, London 1981, Thomas SR and Dixon RL (eds.) NMR in medicine: The instrumentation and clinical applications, Medical Physics Monograph No. 14, American Institute of Physics, New York 1986. Anderson W et al: U.S. Pat. No. 3,475,680, Ernst RR: U.S. Pat. No. 3,501,691, Tomlinson BL et al: U.S. Pat. No. 4 034 191, Ernst RR: U.S. Pat. No. 3,873,909, Ernst RR: U.S. Pat. No. 4 070 611, Bertrand RD et al: U.S. Pat. No. 4,345,207, Young IR: U.S. Pat. No. 4,563,647, Hofer DC et al: U.S. Pat. No. 4,110,681, Savelainen MK: Magnetic resonance imaging at 0.02 T: Design and evaluation of radio frequency coils with wave winding, Acta Polytechnica Scandinavica Ph 158, Helsinki 1988, Sepponen RE: U.S. Pat. No. 4,743,850, Sepponen RE: U.S. Pat. No. 4,654,595, Savelainen MK: U.S. Pat. No. 4,712,068, Sepponen RE: U.S. Pat. No. 4,587,493, Savelainen MK: U.S. Pat. No. 4,644,281 and Kupiainen J: U.S. Pat. No. 4,668,904.
Dynamic nuclear polarization has been described e.g. in the following references: Lepley AR and Closs GL: Chemically induced magnetic polarization, Wiley, New York, 1973, Potenza J: Measurement and Applications of dynamic nuclear polarization, Adv. Mol. Relaxation Processes vol. 4, Elsevier, Amsterdam 1972, pp. 229-354, Ettinger KV: U.S. Pat. No. 4,719,425.
DNP is a magnetic double resonance method which thus requires two separate spin populations. Such spin populations include e.g. The spins of electrons and protons. In a double resonance method, the distribution of one spin population on various energy levels is changed and the other spin population is under observation. When certain conditions are met, the resonance signal of a spin population being observed will increase. The amplified signal may have an amplitude which is several hundred times higher than the non-amplified signal. The amplification factor can be positive or negative. In terms of its characteristics, the amplified signal is highly sensitive to the physico-chemical properties and reactions of a spin environment, so its application for the examination of chemical properties of a material is obvious. The reference Ettinger KV: U.S. Pat. No. 4,719,425 discloses as applications the mapping of concentrations of paramagnetic components and the mapping of the activity of cerebral nerve cells. The references Lurie DJ, Bussel DM, Bell LH, Mallard JR: Proton Electron Double Resonance Imaging: A new method for imaging free radicals, Proc. S.M.R.M. Fifth Annual Meeting, 1987, New York, p. 24 and Lurie DJ, Bussel DM, Bell LH, Mallard JR: Proton-Electron Double Magnetic Resonance Imaging of free radical solutions, J. Magn. Reson., vol. 76, 1988, pp. 366-370 disclose as possible applications the mappings of free radical groups, nitroxide radicals and degree of oxidation.
According to the prior art and referring to FIG. 1, an object P to be examined is placed in a magnetic field B.sub.o +B.sub.o homogeneous as possible (so-called polarizing magnetic field), provided by magnet H and magnetic cell system Ml the apparatus further comprising a signal coil L for detecting an NMR signal connected to an NMR spectrometer including a computer a resonator array R for irradiating the object with a magnetic field having a frequency of electron spin resonance (ESR), said array R being provided with an oscillator and power amplifier unit S, the apparatus including a gradient coil assembly 6 for encoding positional information, the power required thereby being produced by gradient current sources GC controlled by the spectrometer.
In the prior known technology the electron spin system is saturated by irradiating an object at a frequency which matches the ESR frequency in field B.sub.o and by detecting an NMR signal at a frequency which matches the field strength B.sub.o. Thus, for example, the strength 0.04T of B.sub.o used in the above-cited references is matched by the ESR frequency of 1.12 GHz and the NMR frequency of 1.7 MHz.
It is also prior known to effect the saturation of an electron spin system in a different field from the observation of a nuclear magnetic resonance signal. When the diameter of an object is great relative to the wavelength of radiation, which is the case e.g. in the examination of a human body, the alteration of a field brings about considerable safety and technical benefits. Such solution and its benefits are described in references Sepponen RE: FI Pat Appln 883153 and Lurie DJ, Hutchison JMS, Bell LH, Nichclson I, Bussel DM, Mallard JR: Field-Cycled Proton-Electron Double Resonance Imaging of Free Radicals in Large Aqueous Samples, J. Magn. Reson., vol. 84, pp. 431-437, 1989.
A drawback in the prior art is e.g. that, when an object to be examined is relatively large and electrically conductive, the saturation of an electron spin system must be performed at a relatively low frequency and the strength of a polarizing magnetic field must be set relatively low. Thus, the intensity of an NMR signal remains weak unless the strength of a polarizing field is increased for the duration of signal recording. A quick change of the field strength adds to the complexity of the apparatus. It is also difficult to carry out imaging operations with such imaging sequences in which the excitation pulses have a short repetition interval. This type of imaging sequences have been described e.g. in references Frahm J, Haase A, Matthaei D, Haenicke W, Merboldt K-D: U.S. Pat. No. 4,707,658 and Gyngnell ML: U.S. Pat. No. 4,699,148, prior to that in references Tanttu J: Koelaitteisto NMR-kuvausta varten (Test apparatus for NMR imaging), Graduation Thesis, Helsinki Technical University, Department of technical Physics, 1982, p. 69 and Pohjonen J: Koelaitteisto liikuvan kohteen NMR-kuvausta varten (Test apparatus for the NMR imaging of a moving object), Licenciate Thesis, Helsinki Technical University, Department of Technical Physics, 1984, pp. 39-40. Furthermore, it is difficult to perform so-called multiple slice imaging whose principle is disclosed e.g. in reference Crooks LE: Selective irradiation line scan techniques of NMR imaging, IEEE Trans. Nucl. Sci., vol. 27, pp. 1239-1241, 1980.
By means of the invention set forth in the claims it is possible to eliminate the drawbacks of the prior art and to enable the use of multiple slice and short repetition interval imaging sequences in certain applications.